nfsd41: sunrpc: move struct rpc_buffer def into sunrpc.h

Move struct rpc_buffer's definition into a sunrpc.h, a common, internal
header file, in preparation for supporting the nfsv4.1 backchannel.

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
[nfs41: sunrpc: #include <linux/net.h> from sunrpc.h]
Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>